]>
Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
RKF Engineering Solutions, LLC
7500 Old Georgetown RoadSuite 1275BethesdaMD20814-6198United States of AmericaBSipos@rkf-eng.com
University of California, Berkeley
Computer Science Division445 Soda HallBerkeleyCA94720-1776United States of Americademmer@cs.berkeley.edu
Aalto University
Department of Communications and NetworkingPO Box 13000Aalto02015Finlandjo@netlab.tkk.fiQuebecQCCanadasimon@per.reau.ltTransport
Delay Tolerant Networking
This document describes a revised protocol for the TCP-based convergence
layer (TCPCL) for Delay-Tolerant Networking (DTN).
The protocol revision is based on implementation issues in the original
TCPCL Version 3 of RFC7242 and updates to the Bundle Protocol contents,
encodings, and convergence layer requirements in Bundle Protocol Version 7.
Specifically, the TCPCLv4 uses CBOR-encoded BPv7 bundles as its service data
unit being transported and provides a reliable transport of such bundles.
Several new IANA registries are defined for TCPCLv4 which define some
behaviors inherited from TCPCLv3 but with updated encodings and/or semantics.
This document describes the TCP-based convergence-layer protocol for
Delay-Tolerant Networking. Delay-Tolerant Networking is an end-to-
end architecture providing communications in and/or through highly
stressed environments, including those with intermittent
connectivity, long and/or variable delays, and high bit error rates.
More detailed descriptions of the rationale and capabilities of these
networks can be found in "Delay-Tolerant Network Architecture"
.
An important goal of the DTN architecture is to accommodate a wide
range of networking technologies and environments. The protocol used
for DTN communications is the Bundle Protocol Version 7 (BPv7)
,
an
application-layer protocol that is used to construct a store-and-forward
overlay network.
BPv7 requires the services of a "convergence-layer adapter" (CLA)
to send and receive bundles using the service of
some "native" link, network, or Internet protocol.
This document describes one such convergence-layer adapter that uses the
well-known Transmission Control Protocol (TCP).
This convergence layer is referred to as TCP Convergence Layer Version 4 (TCPCLv4).
For the remainder of this document, the abbreviation "BP"
without the version suffix refers to BPv7.
For the remainder of this document, the abbreviation "TCPCL"
without the version suffix refers to TCPCLv4.
The locations of the TCPCL and the BP in the Internet model protocol
stack (described in
) are shown in Figure 1.
In particular, when BP is using TCP as its bearer with TCPCL as its
convergence layer, both BP and TCPCL reside at the application layer
of the Internet model.
+-------------------------+
| DTN Application | -\
+-------------------------| |
| Bundle Protocol (BP) | -> Application Layer
+-------------------------+ |
| TCP Conv. Layer (TCPCL) | |
+-------------------------+ |
| TLS (optional) | -/
+-------------------------+
| TCP | ---> Transport Layer
+-------------------------+
| IPv4/IPv6 | ---> Network Layer
+-------------------------+
| Link-Layer Protocol | ---> Link Layer
+-------------------------+
This document describes the format of the protocol data units passed
between entities participating in TCPCL communications. This
document does not address:
The format of protocol data units of the Bundle Protocol, as those
are defined elsewhere in
and
.
This includes the concept of bundle fragmentation or bundle encapsulation.
The TCPCL transfers bundles as opaque data blocks.
Mechanisms for locating or identifying other bundle entities within
an internet.
This version of the TCPCL provides the following services to support
the overlaying Bundle Protocol agent.
In all cases, this is not an API defintion but a logical description
of how the CL may interact with the BP agent.
Each of these interactions may be associated with any number of
additional metadata items as necessary to support the operation
of the CL or BP agent.
The TCPCL allows a BP agent to pre-emptively attempt to establish
a TCPCL session with a peer entity.
Each session attempt can send a different set of session negotiation
parameters as directed by the BP agent.
The TCPCL allows a BP agent to pre-emptively terminate an established
TCPCL session with a peer entity.
The terminate request is on a per-session basis.
The TCPCL supports indication when the session state changes.
The top-level session states indicated are:
A TCP connection has been made (as either active or passive entity) and contact negotiation has begun.Contact negotation has been completed (including possible TLS use) and session negotiation has begun.The session has been fully established and is ready for its first transfer.The entity received a SESS_TERM message and is in the closing state.The session has finished normal termination sequencing..The session ended without normal termination sequencing.
The TCPCL supports indication when the live/idle sub-state changes.
This occurs only when the top-level session state is Established.
Because TCPCL transmits serially over a TCP connection, it suffers
from "head of queue blocking" this indication provides
information about when a session is available for
immediate transfer start.
The principal purpose of the TCPCL is to allow a BP agent to
transmit bundle data over an established TCPCL session.
Transmission request is on a per-session basis, the CL does not
necessarily perform any per-session or inter-session queueing.
Any queueing of transmissions is the obligation of the BP agent.
The TCPCL supports positive indication when a bundle has been fully
transferred to a peer entity.
The TCPCL supports positive indication of intermediate progress
of transferr to a peer entity.
This intermediate progress is at the granularity of each
transferred segment.
The TCPCL supports positive indication of certain reasons for
bundle transmission failure, notably when the peer entity rejects
the bundle or when a TCPCL session ends before transferr success.
The TCPCL itself does not have a notion of transfer timeout.
The TCPCL supports indication to the reciver just before any
transmssion data is sent.
This corresponds to reception of the XFER_SEGMENT message with
the START flag set.
The TCPCL allows a BP agent to interrupt an individual transfer
before it has fully completed (successfully or not).
Interruption can occur any time after the reception is initialized.
The TCPCL supports positive indication when a bundle has been fully
transferred from a peer entity.
The TCPCL supports positive indication of intermediate progress
of transfer from the peer entity.
This intermediate progress is at the granularity of each
transferred segment.
Intermediate reception indication allows a BP agent the chance
to inspect bundle header contents before the entire bundle is
available, and thus supports the
"Reception Interruption" capability.
The TCPCL supports positive indication of certain reasons for
reception failure, notably when the local entity rejects an attempted
transfer for some local policy reason or when a TCPCL session
ends before transfer success.
The TCPCL itself does not have a notion of transfer timeout.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in
.
This section contains definitions specific to the TCPCL protocol.
This is the notional TCPCL application that initiates TCPCL sessions.
This design, implementation, configuration, and specific behavior of such
an entity is outside of the scope of this document.
However, the concept of an entity has utility within the scope of
this document as the container and initiator of TCPCL sessions.
The relationship between a TCPCL entity and TCPCL sessions is defined as follows:
A TCPCL Entity MAY actively initiate any number of TCPCL Sessions and should do so
whenever the entity is the initial transmitter of information to another entity in the network.
A TCPCL Entity MAY support zero or more passive listening elements that listen for
connection requests from other TCPCL Entities operating on other entitys in the network.
A TCPCL Entity MAY passivley initiate any number of TCPCL Sessions from requests received
by its passive listening element(s) if the entity uses such elements.
These relationships are illustrated in .
For most TCPCL behavior within a session, the two entities are
symmetric and there is no protocol distinction between them.
Some specific behavior, particularly during session establishment, distinguishes
between the active entity and the passive entity.
For the remainder of this document, the term "entity"
without the prefix "TCPCL" refers to a TCPCL entity.
The term Connection in this specification exclusively refers to a TCP connection
and any and all behaviors, sessions, and other states association with that TCP connection.
A TCPCL session (as opposed to a TCP connection) is a TCPCL
communication relationship between two TCPCL entities.
Within a single TCPCL session there are two possible transfer streams;
one in each direction, with one stream from each entity being the outbound
stream and the other being the inbound stream.
The lifetime of a TCPCL session is bound to the lifetime of an underlying TCP connection.
A TCPCL session is terminated when the TCP
connection ends, due either to one or both entities actively
terminating the TCP connection or due to network errors causing
a failure of the TCP connection.
For the remainder of this document, the term "session"
without the prefix "TCPCL" refers to a TCPCL session.
These are a set of
values used to affect the operation of the TCPCL for a given
session. The manner in which these parameters are conveyed
to the bundle entity and thereby to the TCPCL is implementation
dependent. However, the mechanism by which two entities
exchange and negotiate the values to be used for a given session
is described in
.
A Transfer stream is a uni-directional user-data path within a TCPCL Session.
Messages sent over a transfer stream are serialized, meaning that one set of
user data must complete its transmission prior to another set of user data
being transmitted over the same transfer stream.
Each uni-directional stream has a single sender entity and a single receiver entity.
This refers to the procedures and mechanisms
for conveyance of an individual bundle from one node to
another.
Each transfer within TCPCL is identified by a Transfer ID number
which is unique only to a single direction within a single Session.
A subset of a transfer of user data being communicated over a trasnfer stream.
A TCPCL session is idle while the only messages being transmitted or received
are KEEPALIVE messages.
A TCPCL session is live while any messages are being transmitted or received.
The TCPCL uses numeric codes to encode specific reasons for individual
failure/error message types.
The relationship between connections, sessions, and streams is shown in .
+--------------------------------------------+
| TCPCL Entity |
| | +----------------+
| +--------------------------------+ | | |-+
| | Actively Inititated Session #1 +------------->| Other | |
| +--------------------------------+ | | TCPCL Entity's | |
| ... | | Passive | |
| +--------------------------------+ | | Listener | |
| | Actively Inititated Session #n +------------->| | |
| +--------------------------------+ | +----------------+ |
| | +-----------------+
| +---------------------------+ |
| +---| +---------------------------+ | +----------------+
| | | | Optional Passive | | | |-+
| | +-| Listener(s) +<-------------+ | |
| | +---------------------------+ | | | |
| | | | Other | |
| | +---------------------------------+ | | TCPCL Entity's | |
| +--->| Passively Inititated Session #1 +-------->| Active | |
| | +---------------------------------+ | | Initiator(s) | |
| | | | | |
| | +---------------------------------+ | | | |
| +--->| Passively Inititated Session #n +-------->| | |
| +---------------------------------+ | +----------------+ |
| | +-----------------+
+--------------------------------------------+
+----------------------------+ +--------------------------+
| TCPCL Session | | TCPCL "Other" Session |
| | | |
| +-----------------------+ | | +---------------------+ |
| | TCP Connection | | | | TCP Connection | |
| | | | | | | |
| | +-------------------+ | | | | +-----------------+ | |
| | | Optional Inbound | | | | | | Peer Outbound | | |
| | | Transfer Stream |<-[Seg]--[Seg]--[Seg]-| | Transfer Stream | | |
| | | ----- | | | | | | ----- | | |
| | | RECEIVER | | | | | | SENDER | | |
| | +-------------------+ | | | | +-----------------+ | |
| | | | | | | |
| | +-------------------+ | | | | +-----------------+ | |
| | | Optional Outbound | | | | | | Peer Inbound | | |
| | | Transfer Stream |------[Seg]---[Seg]---->| Transfer Stream | | |
| | | ----- | | | | | | ----- | | |
| | | SENDER | | | | | | RECEIVER | | |
| | +-------------------+ | | | | +-----------------+ | |
| +-----------------------+ | | +---------------------+ |
+----------------------------+ +--------------------------+
The service of this protocol is the transmission of DTN bundles via
the Transmission Control Protocol (TCP).
This document specifies the encapsulation of bundles,
procedures for TCP setup and teardown, and a set of messages and node
requirements.
The general operation of the protocol is as follows.
First, one node establishes a TCPCL session to the other by
initiating a TCP connection in accordance with
.
After setup of the TCP connection is
complete, an initial contact header is exchanged in both directions
to establish a shared TCPCL version and possibly initiate TLS security.
Once contact negotiation is complete, TCPCL messaging is available and
the session negotiation is used to set parameters of the TCPCL session.
One of these parameters is a singleton
endpoint identifier for each node (not the singleton Endpoint
Identifier (EID) of any application running on the node) to denote
the bundle-layer identity of each DTN node. This is used to assist
in routing and forwarding messages (e.g. to prevent loops).
Once negotiated, the parameters of a TCPCL session cannot change
and if there is a desire by either peer to transfer data under
different parameters then a new session must be established.
This makes CL logic simpler but relies on the assumption that
establishing a TCP connection is lightweight enough that TCP
connection overhead is negligable compared to TCPCL data sizes.
Once the TCPCL session is established and configured in this way,
bundles can be transferred in either direction.
Each transfer is performed by an sequence of
logical segments of data within XFER_SEGMENT messages.
Multiple bundles can be transmitted consecutively in a single
direction on a single TCPCL connection.
Segments from different bundles are never interleaved.
Bundle interleaving can be
accomplished by fragmentation at the BP layer or by establishing multiple
TCPCL sessions between the same peers.
A feature of this protocol is for the receiving node to send
acknowledgment (XFER_ACK) messages as bundle data segments arrive. The
rationale behind these acknowledgments is to enable the sender node
to determine how much of the bundle has been received, so that in
case the session is interrupted, it can perform reactive
fragmentation to avoid re-sending the already transmitted part of the
bundle.
In addition, there is no explicit flow control on the TCPCL
layer.
A TCPCL receiver can interrupt the
transmission of a bundle at any point in time by replying with a
XFER_REFUSE message, which causes the sender to stop transmission
of the associated bundle (if it hasn't already finished transmission)
Note: This enables a cross-layer optimization in
that it allows a receiver that detects that it already has received a
certain bundle to interrupt transmission as early as possible and
thus save transmission capacity for other bundles.
For sessions that are idle, a KEEPALIVE message is
sent at a negotiated interval.
This is used to convey node live-ness information during otherwise
message-less time intervals.
A SESS_TERM message is used to start the closing of a TCPCL session
(see
).
During shutdown sequencing, in-progress transfers can be completed but no
new transfers can be initiated.
A SESS_TERM message can also be used to refuse a session setup by a
peer (see
).
It is an implementation matter to determine whether or not to close a
TCPCL session while there are no transfers queued or in-progress.
Once a session is established established,
TCPCL is a symmetric protocol between the peers.
Both sides can start sending data segments in a session, and one
side's bundle transfer does not have to complete before the other
side can start sending data segments on its own. Hence, the protocol
allows for a bi-directional mode of communication.
Note that in the case of concurrent bidirectional transmission,
acknowledgment segments MAY be interleaved with data segments.
The states of a nominal TCPCL session (i.e. without session failures)
are indicated in .
+-------+
| START |
+-------+
|
TCP Establishment
|
V
+-----------+ +---------------------+
| TCP |----------->| Contact / Session |
| Connected | | Negotiation |
+-----------+ +---------------------+
|
+-----Session Parameters-----+
| Negotiated
V
+-------------+ +-------------+
| Established |----New Transfer---->| Established |
| Session | | Session |
| Idle |<---Transfers Done---| Live |
+-------------+ +-------------+
| |
+------------------------------------+
|
SESS_TERM Exchange
|
V
+-------------+
| Established | +-------------+
| Session |----Transfers------>| TCP |
| Ending | Done | Terminating |
+-------------+ +-------------+
|
+------------Close Message------------+
|
V
+-------+
| END |
+-------+
Notes on Established Session states:
Session "Live" means transmitting or reeiving over a transfer stream.Session "Idle" means no transmission/reception over a transfer stream.Session "Closing" means no new transfers will be allowed.
The contact negotiation sequencing is performed either as the
active or passive peer, and is illustrated in
and respectively which both share the data
validation and analyze final states of .
+-------+
| START |-----TCP-----+
+-------+ Connecting |
V
+-----------+ +---------+
| Connected |--OK-->| Send CH |--OK-->[PCH]
+-----------+ +---------+
| |
Error Error
| |
V |
[TCPTERM]<-------------+
+-------+
| START |-----TCP----->[PCH]
+-------+ Connected
+-------->[TCPTERM]<----------+
| |
Timeout Error
or Error |
| |
+-------+ +---------+ Contact +----------+
| START |---->| Waiting |---- Header --->| Validate |
+-------+ +---------+ Received +----------+
|
+---------------------------+
|
V
+---------+
+--Error--| Analyze |---No TLS---->[SI]
| | | ^
| +---------+ |
| | |
V TLS |
[TCPTERM] Negotiated |
^ | |
| V |
| +-----------+ |
| | Establish |---Success---+
+--Error--| TLS |
+-----------+
The session negotiation sequencing is performed either as the
active or passive peer, and is illustrated in
and respectively which both share the data
validation and analyze final states of .
+-------+ TCPCL
| START |--Messaging--+
+-------+ Available |
V
+----------------+
| Send SESS_INIT |--OK-->[PSI]
+----------------+
|
Error
|
V
[SESSTERM]
+-------+ TCPCL
| START |---Messaging-->[PSI]
+-------+ Available
+------->[SESSTERM]<--------+
| |
Timeout Error
or Error |
| |
+-------+ +---------+ +----------+
| START |---->| Waiting |---SESS_INIT--->| Validate |
+-------+ +---------+ Received +----------+
|
+---------------------------+
|
V
+---------+ +--------------+
+--Error--| Analyze |---->| Established |
| | | | Session Idle |
| +---------+ +--------------+
V
[SESSTERM]
Transfers can occur after a session is established and it's not
in the ending state.
Each transfer occurs within a single logical transfer stream
between a sender and a receiver, as illustrated in
and
respectively.
+--Send XFER_SEGMENT--+
+--------+ | |
| Stream | +-------------+ |
| Idle |---Send XFER_SEGMENT-->| In Progress |<------------+
+--------+ +-------------+
|
+---------All segments sent-------+
|
V
+---------+ +--------+
| Waiting |---- Receive Final---->| Stream |
| for Ack | XFER_ACK | IDLE |
+---------+ +--------+
Notes on transfer sending:
Pipelining of transfers can occur when the sending entity begins a new transfer while in the "Waiting for Ack" state.
+-Receive XFER_SEGMENT-+
+--------+ | Send XFER_ACK |
| Stream | +-------------+ |
| IDLE |--Receive XFER_SEGMENT-->| In Progress |<-------------+
+--------+ +-------------+
|
+--------Sent Final XFER_ACK--------+
|
V
+--------+
| Stream |
| IDLE |
+--------+
Each TCPCL session allows a negotiated transfer segmentation polcy
to be applied in each transfer direction.
A receiving node can set the Segment MRU in its contact header to
determine the largest acceptable segment size, and a transmitting
node can segment a transfer into any sizes smaller than the
receiver's Segment MRU.
It is a network administration matter to determine an appropriate
segmentation policy for entities operating TCPCL, but guidance given
here can be used to steer policy toward performance goals.
It is also advised to consider the Segment MRU in relation to
chunking/packetization performed by TLS, TCP, and any intermediate
network-layer nodes.
For a simple network expected to exchange relatively small bundles,
the Segment MRU can be set to be identical to the Transfer MRU which
indicates that all transfers can be sent with a single data
segment (i.e. no actual segmentation).
If the network is closed and all transmitters are known to follow
a single-segment transfer policy, then receivers can avoid the
necessity of segment reassembly.
Because this CL operates over a TCP stream, which suffers from
a form of head-of-queue blocking between messages, while one
node is transmitting a single XFER_SEGMENT message it is not
able to transmit any XFER_ACK or XFER_REFUSE for any associated
received transfers.
In situations where the maximum message size is desired to be
well-controlled, the
Segment MRU can be set to the largest acceptable size (the
message size less XFER_SEGMENT header size) and transmitters
can always segment a transfer into maximum-size chunks no larger
than the Segment MRU.
This guarantees that any single XFER_SEGMENT will not monopolize
the TCP stream for too long, which would prevent outgoing
XFER_ACK and XFER_REFUSE associated with received transfers.
Even after negotiation of a Segment MRU for each receiving node,
the actual transfer segmentation only needs to guarantee than
any individual segment is no larger than that MRU.
In a situation where network "goodput" is dynamic, the transfer
segmentation size can also be dynamic in order to control
message transmission duration.
Many other policies can be established in a TCPCL network between
these two extremes.
Different policies can be applied to each direction to/from any
particular node.
Additionally, future header and transfer extension types can apply
further nuance to transfer policies and policy negotiation.
The following figure depicts the protocol exchange for a
simple session, showing the session establishment and the
transmission of a single bundle split into three data segments (of
lengths "L1", "L2", and "L3") from Entity A to Entity B.
Note that the sending node can transmit multiple XFER_SEGMENT
messages without waiting for the corresponding
XFER_ACK responses. This enables pipelining of messages on a
transfer stream. Although this example only demonstrates a single bundle
transmission, it is also possible to pipeline multiple XFER_SEGMENT
messages for different bundles without necessarily waiting for
XFER_ACK messages to be returned for each one. However,
interleaving data segments from different bundles is not allowed.
No errors or rejections are shown in this example.
Entity A Entity B
======== ========
+-------------------------+
| Contact Header | -> +-------------------------+
+-------------------------+ <- | Contact Header |
+-------------------------+
+-------------------------+
| SESS_INIT | -> +-------------------------+
+-------------------------+ <- | SESS_INIT |
+-------------------------+
+-------------------------+
| XFER_SEGMENT (start) | ->
| Transfer ID [I1] |
| Length [L1] |
| Bundle Data 0..(L1-1) |
+-------------------------+
+-------------------------+ +-------------------------+
| XFER_SEGMENT | -> <- | XFER_ACK (start) |
| Transfer ID [I1] | | Transfer ID [I1] |
| Length [L2] | | Length [L1] |
|Bundle Data L1..(L1+L2-1)| +-------------------------+
+-------------------------+
+-------------------------+ +-------------------------+
| XFER_SEGMENT (end) | -> <- | XFER_ACK |
| Transfer ID [I1] | | Transfer ID [I1] |
| Length [L3] | | Length [L1+L2] |
|Bundle Data | +-------------------------+
| (L1+L2)..(L1+L2+L3-1)|
+-------------------------+
+-------------------------+
<- | XFER_ACK (end) |
| Transfer ID [I1] |
| Length [L1+L2+L3] |
+-------------------------+
+-------------------------+
| SESS_TERM | -> +-------------------------+
+-------------------------+ <- | SESS_TERM |
+-------------------------+
For bundle transmissions to occur using the TCPCL, a TCPCL session
MUST first be established between communicating entities. It is up to
the implementation to decide how and when session setup is
triggered. For example, some sessions MAY be opened proactively
and maintained for as long as is possible given the network
conditions, while other sessions MAY be opened only when there is
a bundle that is queued for transmission and the routing algorithm
selects a certain next-hop node.
To establish a TCPCL session, an entity MUST first establish a TCP
connection with the intended peer entity, typically by using the
services provided by the operating system.
Destination port number 4556 has been assigned by IANA as the Registered
Port number for the TCP convergence layer.
Other destination port numbers MAY be used per local configuration.
Determining a peer's destination port number (if different from the
registered TCPCL port number) is up to the implementation.
Any source port number MAY be used for TCPCL sessions.
Typically an operating system assigned number in the TCP Ephemeral range
(49152-65535) is used.
If the entity is unable to establish a TCP connection for any reason,
then it is an implementation matter to determine how to handle the
connection failure. An entity MAY decide to re-attempt to establish the
connection. If it does so, it MUST NOT overwhelm its target with
repeated connection attempts. Therefore, the entity MUST retry the
connection setup no earlier than some delay time from the last attempt,
and it SHOULD use a (binary) exponential backoff
mechanism to increase this delay in case of repeated failures.
Once a TCP connection is established, each entity MUST immediately
transmit a contact header over the TCP connection. The format of the
contact header is described in
.
Once a TCP connection is established, both parties exchange a contact
header. This section describes the format of the contact header and
the meaning of its fields.
Upon receipt of the contact header, both entities perform the validation
and negotiation procedures defined in
.
After receiving the contact header from the other entity, either entity
MAY refuse the session by sending a SESS_TERM message with an appropriate
reason code.
The format for the Contact Header is as follows:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| magic='dtn!' |
+---------------+---------------+---------------+---------------+
| Version | Flags |
+---------------+---------------+
See
for details on the use of each
of these contact header fields.
The fields of the contact header are:
A four-octet field that always contains the octet sequence 0x64
0x74 0x6e 0x21, i.e., the text string "dtn!" in US-ASCII
(and UTF-8).
A one-octet field value containing the value 4 (current
version of the protocol).
A one-octet field of single-bit flags, interpreted according to the
descriptions in
.
NameCodeDescriptionCAN_TLS0x01If bit is set, indicates that the sending peer is capable of TLS security.Reservedothers
Upon reception of the contact header, each node follows the following
procedures to ensure the validity of the TCPCL session and to
negotiate values for the session parameters.
If the magic string is not present or is not valid, the connection
MUST be terminated. The intent of the magic string is to provide
some protection against an inadvertent TCP connection by a different
protocol than the one described in this document. To prevent a flood
of repeated connections from a misconfigured application, an entity MAY
elect to hold an invalid connection open and idle for some time
before closing it.
The first negotiation is on the TCPCL protocol version to use.
The active node always sends its Contact Header first and waits for
a response from the passive node.
The active node can repeatedly attempt different protocol versions
in descending order until the passive node accepts one with a
corresponding Contact Header reply.
Only upon response of a Contact Header from the passive node is
the TCPCL protocol version established and parameter negotiation begun.
During contact initiation, the active TCPCL node SHALL send the highest TCPCL protocol
version on a first session attempt for a TCPCL peer.
If the active node receives a Contact Header with a different
protocol version than the one sent earlier on the TCP connection,
the TCP connection SHALL be terminated.
If the active node receives a SESS_TERM message with reason of
"Version Mismatch", that node MAY attempt further TCPCL sessions with the
peer using earlier protocol version numbers in decreasing order.
Managing multi-TCPCL-session state such as this is an implementation matter.
If the passive node receives a contact header containing a version that is
greater than the current version of the protocol that the node
implements, then the node SHALL shutdown the session with a reason code
of "Version mismatch".
If the passive node receives a contact header with a
version that is lower than the version of the protocol that the node
implements, the node MAY either terminate the session (with a reason code
of "Version mismatch") or the node MAY adapt its operation to
conform to the older version of the protocol.
The decision of version fall-back is an implementation matter.
This version of the TCPCL supports establishing a Transport
Layer Security (TLS) session within an existing TCP connection.
When TLS is used within the TCPCL it affects the entire session.
Once established, there is no mechanism available to downgrade a
TCPCL session to non-TLS operation.
If this is desired, the entire TCPCL session MUST be terminated and a new
non-TLS-negotiated session established.
The use of TLS is negotated using the Contact Header as described
in
.
After negotiating an Enable TLS parameter of true, and before any other
TCPCL messages are sent within the session, the session entities SHALL
begin a TLS handshake in accordance with
.
The parameters within each TLS negotiation are implementation dependent but
any TCPCL node SHALL follow all recommended practices of
, or any updates or successors that
become part of .
By convention, this protocol uses the node which initiated the
underlying TCP connection as the "client" role
of the TLS handshake request.
The TLS handshake, if it occurs, is considered to be part of the contact
negotiation before the TCPCL session itself is established.
Specifics about sensitive data exposure are discussed in
.
If a TLS handshake cannot negotiate a TLS session, both entities of the TCPCL
session SHALL terminate the TCP connection.
At this point the TCPCL session has not yet been established so there
is no TCPCL session to terminate.
This also avoids any potential security issues assoicated with
further TCP communication with an untrusted peer.
After a TLS session is successfully established, the active peer
SHALL send a SESS_INIT message to begin session negotiation.
This session negotation and all subsequent messaging are secured.
A summary of a typical CAN_TLS usage is shown in the sequence in
below.
Entity A Entity B
======== ========
+-------------------------+
| Open TCP Connnection | ->
+-------------------------+ +-------------------------+
<- | Accept Connection |
+-------------------------+
+-------------------------+
| Contact Header | ->
+-------------------------+ +-------------------------+
<- | Contact Header |
+-------------------------+
+-------------------------+ +-------------------------+
| TLS Negotiation | -> <- | TLS Negotiation |
| (as client) | | (as server) |
+-------------------------+ +-------------------------+
... secured TCPCL messaging, starting with SESS_INIT ...
+-------------------------+ +-------------------------+
| SESS_TERM | -> <- | SESS_TERM |
+-------------------------+ +-------------------------+
After the initial exchange of a contact header, all messages
transmitted over the session are identified by a one-octet header
with the following structure:
0 1 2 3 4 5 6 7
+---------------+
| Message Type |
+---------------+
The message header fields are as follows:
Indicates the type of the message as per
below.
Encoded values are listed in
.
TypeDescriptionSESS_INIT
Contains the session parameter inputs from one of the entities,
as described in .
XFER_SEGMENT
Indicates the transmission of a segment of bundle data, as described in
.
XFER_ACK
Acknowledges reception of a data segment, as described in
.
XFER_REFUSE
Indicates that the transmission of the current bundle SHALL be stopped, as described in
.
KEEPALIVE
Used to keep TCPCL session active, as described in
.
SESS_TERM
Indicates that one of the entities participating in the session wishes to cleanly terminate the session, as described in
.
MSG_REJECT
Contains a TCPCL message rejection, as described in
.
Before a session is established and ready to transfer bundles, the
session parameters are negotiated between the connected entities.
The SESS_INIT message is used to convey the per-entity parameters
which are used together to negotiate the per-session parameters
as described in .
The format of a SESS_INIT message is as follows in
.
+-------------------------------+
| Message Header |
+-------------------------------+
| Keepalive Interval (U16) |
+-------------------------------+
| Segment MRU (U64) |
+-------------------------------+
| Transfer MRU (U64) |
+-------------------------------+
| EID Length (U16) |
+-------------------------------+
| EID Data (variable) |
+-------------------------------+
| Session Extension Length (U32)|
+-------------------------------+
| Session Extension Items (var.)|
+-------------------------------+
The fields of the SESS_INIT message are:
A 16-bit unsigned integer indicating the interval,
in seconds, between any subsequent messages being transmitted by the peer.
The peer receiving this contact header uses this interval to determine
how long to wait after any last-message transmission and a necessary
subsequent KEEPALIVE message transmission.
A 64-bit unsigned integer indicating the largest allowable single-segment
data payload size to be received in this session.
Any XFER_SEGMENT sent to this peer SHALL have a data payload no longer
than the peer's Segment MRU.
The two entities of a single session MAY have different Segment MRUs, and
no relation between the two is required.
A 64-bit unsigned integer indicating the largest allowable total-bundle
data size to be received in this session.
Any bundle transfer sent to this peer SHALL have a Total Bundle Length
payload no longer than the peer's Transfer MRU.
This value can be used to perform proactive bundle fragmentation.
The two entities of a single session MAY have different Transfer MRUs, and
no relation between the two is required.
Together these fields represent a variable-length text string.
The EID Length is a 16-bit unsigned integer indicating the number of
octets of EID Data to follow.
A zero EID Length SHALL be used to indicate the lack of EID rather than a
truly empty EID.
This case allows an entity to avoid exposing EID information on an
untrusted network.
A non-zero-length EID Data SHALL contain the UTF-8 encoded EID of some
singleton endpoint in which the sending entity is a member, in the canonical
format of <scheme name>:<scheme-specific part>.
This EID encoding is consistent with
.
Together these fields represent protocol extension data
not defined by this specification.
The Session Extension Length is the total number of octets to follow which
are used to encode the Session Extension Item list.
The encoding of each Session Extension Item is within a consistent data
container as described in
.
The full set of Session Extension Items apply for the duration of
the TCPCL session to follow.
The order and mulitplicity of these Session Extension Items MAY be
significant, as defined in the associated type specification(s).
An entity calculates the parameters for a TCPCL session by
negotiating the values from its own preferences (conveyed by the
contact header it sent to the peer) with the preferences of the peer node
(expressed in the contact header that it received from the peer).
The negotiated parameters defined by this specification are
described in the following paragraphs.
The maximum transmit unit (MTU) for whole transfers and individual segments
are idententical to the Transfer MRU and Segment MRU, respectively,
of the recevied contact header.
A transmitting peer can send individual segments with any size smaller
than the Segment MTU, depending on local policy, dynamic network
conditions, etc.
Determining the size of each transmitted segment is an implementation
matter.
Negotiation of the Session Keepalive parameter is performed by taking
the minimum of this two contact headers' Keepalive Interval.
The Session Keepalive interval is a parameter for the behavior described
in
.
Negotiation of the Enable TLS parameter is performed by taking
the logical AND of the two contact headers' CAN_TLS flags.
A local security policy is then applied to determine of the negotated value
of Enable TLS is acceptable.
It can be a reasonable security policy to both require or disallow
the use of TLS depending upon the desired network flows.
If the Enable TLS state is unacceptable, the node SHALL terminate
the session with a reason code of "Contact Failure".
Note that this contact failure is different than a failure of TLS handshake
after an agreed-upon and acceptable Enable TLS state.
If the negotiated Enable TLS value is true and acceptable then
TLS negotiation feature (described in
) begins
immediately following the contact header exchange.
Once this process of parameter negotiation is completed (which includes a
possible completed TLS handshake of the connection to use TLS),
this protocol
defines no additional mechanism to change the parameters of an
established session; to effect such a change, the TCPCL session MUST
be terminated and a new session established.
Each of the Session Extension Items SHALL be encoded in an identical
Type-Length-Value (TLV) container form as indicated in
.
The fields of the Session Extension Item are:
A one-octet field containing generic bit flags about the Item,
which are listed in
.
If a TCPCL entity receives a Session Extension Item with an unknown Item Type
and the CRITICAL flag set, the entity SHALL close the
TCPCL session with SESS_TERM reason code of "Contact Failure".
If the CRITICAL flag is not set, an entity SHALL skip over and ignore
any item with an unknown Item Type.
A 16-bit unsigned integer field containing the type of the extension item.
This specification does not define any extension types directly, but does
allocate an IANA registry for such codes
(see ).
A 32-bit unsigned integer field containing the number of Item Value octets
to follow.
A variable-length data field which is interpreted according to the
associated Item Type.
This specification places no restrictions on an extension's use of
available Item Value data.
Extension specifications SHOULD avoid the use of large data lengths,
as no bundle transfers can begin until
the full extension data is sent.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Item Flags | Item Type | Item Length...|
+---------------+---------------+---------------+---------------+
| length contd. | Item Value... |
+---------------+---------------+---------------+---------------+
| value contd. |
+---------------+---------------+---------------+---------------+
NameCodeDescriptionCRITICAL0x01If bit is set, indicates that the receiving peer must handle the extension item.Reservedothers
This section describes the protocol operation for the duration of an
established session, including the mechanism for transmitting
bundles over the session.
The protocol includes a provision for transmission of KEEPALIVE
messages over the TCPCL session to help determine if the underlying
TCP connection has been disrupted.
As described in
,
a negotiated parameter of each session is the Session Keepalive interval.
If the negotiated Session Keepalive is zero (i.e. one or both contact
headers contains a zero Keepalive Interval), then the keepalive feature
is disabled.
There is no logical minimum value for the keepalive interval, but when
used for many sessions on an open, shared network a short interval could
lead to excessive traffic.
For shared network use, entities SHOULD choose a keepalive interval
no shorter than 30 seconds.
There is no logical maximum value for the keepalive interval, but an
idle TCP connection is liable for closure by the host operating system
if the keepalive time is longer than tens-of-minutes.
Entities SHOULD choose a keepalive interval no longer than 10 minutes
(600 seconds).
Note: The Keepalive Interval SHOULD NOT be chosen too short as TCP
retransmissions MAY occur in case of packet loss. Those will have to
be triggered by a timeout (TCP retransmission timeout (RTO)), which
is dependent on the measured RTT for the TCP connection so that
KEEPALIVE messages MAY experience noticeable latency.
The format of a KEEPALIVE message is a one-octet message type code of
KEEPALIVE (as described in
) with no additional data.
Both sides SHALL send a KEEPALIVE message whenever the negotiated
interval has elapsed with no transmission of any message (KEEPALIVE
or other).
If no message (KEEPALIVE or other) has been received in a session after some
implementation-defined time duration, then the node SHALL terminate the
session by transmitting a SESS_TERM message (as described in
) with reason code "Idle Timeout".
If configurable, the idle timeout duration SHOULD be no shorter
than twice the keepalive interval.
If not configurable, the idle timeout duration SHOULD be exactly
twice the keepout interval.
If a TCPCL node receives a message which is unknown to it (possibly due
to an unhandled protocol mismatch) or is inappropriate for the current
session state (e.g. a KEEPALIVE message received after contact header
negotiation has disabled that feature), there is a protocol-level
message to signal this condition in the form of a MSG_REJECT reply.
The format of a MSG_REJECT message is as follows in
.
+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Rejected Message Header |
+-----------------------------+
The fields of the MSG_REJECT message are:
A one-octet refusal reason code interpreted according to the
descriptions in
.
The Rejected Message Header is a copy of the Message Header to which the
MSG_REJECT message is sent as a response.
NameCodeDescriptionMessage Type Unknown0x01A message was received with
a Message Type code unknown to the TCPCL node.Message Unsupported0x02A message was received but
the TCPCL node cannot comply with the message contents.Message Unexpected0x03A message was received while the
session is in a state in which the message is not expected.
All of the messages in this section are directly associated with transferring
a bundle between TCPCL entities.
A single TCPCL transfer results in a bundle (handled by the convergence layer
as opaque data) being exchanged from one node to the other.
In TCPCL a transfer is accomplished by dividing a single bundle up into
"segments" based on the receiving-side Segment MRU
(see
).
The choice of the length to use for segments is an implementation matter,
but each segment MUST be no larger than the receiving node's maximum
receive unit (MRU)
(see the field "Segment MRU" of
).
The first segment for a bundle MUST set the 'START' flag,
and the last one MUST set the 'end' flag in the XFER_SEGMENT message flags.
A single transfer (and by extension a single segment) SHALL NOT contain data
of more than a single bundle.
This requirement is imposed on the agent using the TCPCL rather than TCPCL itself.
If multiple bundles are transmitted on a single TCPCL connection,
they MUST be transmitted consecutively without interleaving of segments from multiple bundles.
Each of the bundle transfer messages contains a Transfer ID which is
used to correlate messages (from both sides of a transfer) for each bundle.
A Transfer ID does not attempt to address uniqueness of the bundle data itself
and has no relation to concepts such as bundle fragmentation.
Each invocation of TCPCL by the bundle protocol agent, requesting transmission
of a bundle (fragmentary or otherwise), results in the initiation of a single
TCPCL transfer. Each transfer entails the sending of a sequence of
some number of XFER_SEGMENT and XFER_ACK messages; all are correlated
by the same Transfer ID.
Transfer IDs from each node SHALL be unique within a single TCPCL session.
The initial Transfer ID from each node SHALL have value zero.
Subsequent Transfer ID values SHALL be incremented from the prior Transfer ID
value by one.
Upon exhaustion of the entire 64-bit Transfer ID space, the sending node
SHALL terminate the session with SESS_TERM reason code "Resource Exhaustion".
For bidirectional bundle transfers, a TCPCL node SHOULD NOT rely on any
relation between Transfer IDs originating from each side of the TCPCL session.
Each bundle is transmitted in one or more data segments.
The format of a XFER_SEGMENT message follows in
.
+------------------------------+
| Message Header |
+------------------------------+
| Message Flags (U8) |
+------------------------------+
| Transfer ID (U64) |
+------------------------------+
| Transfer Extension |
| Length (U32) |
| (only for START segment) |
+------------------------------+
| Transfer Extension |
| Items (var.) |
| (only for START segment) |
+------------------------------+
| Data length (U64) |
+------------------------------+
| Data contents (octet string) |
+------------------------------+
The fields of the XFER_SEGMENT message are:
A one-octet field of single-bit flags, interpreted according to the
descriptions in
.
A 64-bit unsigned integer identifying the transfer being made.
Together these fields represent protocol extension data
for this specification.
The Transfer Extension Length and Transfer Extension Item fields
SHALL only be present when the 'START' flag is set on the message.
The Transfer Extension Length is the total number of octets to follow which
are used to encode the Transfer Extension Item list.
The encoding of each Transfer Extension Item is within a consistent data
container as described in
.
The full set of transfer extension items apply only to the
assoicated single transfer.
The order and mulitplicity of these transfer extension items MAY be
significant, as defined in the associated type specification(s).
A 64-bit unsigned integer indicating the number of octets in the
Data contents to follow.
The variable-length data payload of the message.
NameCodeDescriptionEND0x01If bit is set, indicates that this is the last segment of the transfer.START0x02If bit is set, indicates that this is the first segment of the transfer.Reservedothers
The flags portion of the message contains two optional
values in the two low-order bits, denoted 'START' and 'END' in
.
The 'START' bit MUST be set to one if it precedes the transmission of the first
segment of a transfer.
The 'END' bit MUST be set to one when transmitting the last segment of a transfer.
In the case where an entire transfer is accomplished in a single segment,
both the 'START' and 'END' bits MUST be set to one.
Once a transfer of a bundle has commenced, the node MUST only
send segments containing sequential portions of that bundle until it
sends a segment with the 'END' bit set.
No interleaving of multiple transfers from the same node is possible
within a single TCPCL session.
Simultaneous transfers between two entities MAY be achieved using multiple
TCPCL sessions.
Although the TCP transport provides reliable transfer of data between
transport peers, the typical BSD sockets interface provides no means
to inform a sending application of when the receiving application has
processed some amount of transmitted data.
Thus, after transmitting some data, the TCPCL needs an additional mechanism to
determine whether the receiving agent has successfully received the
segment.
To this end, the TCPCL protocol provides feedback messaging whereby a
receiving node transmits acknowledgments of reception of data
segments.
The format of an XFER_ACK message follows in
.
+-----------------------------+
| Message Header |
+-----------------------------+
| Message Flags (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
| Acknowledged length (U64) |
+-----------------------------+
The fields of the XFER_ACK message are:
A one-octet field of single-bit flags, interpreted according to the
descriptions in
.
A 64-bit unsigned integer identifying the transfer being acknowledged.
A 64-bit unsigned integer indicating the total number of octets in the
transfer which are being acknowledged.
A receiving TCPCL node SHALL send an XFER_ACK message in response to
each received XFER_SEGMENT message.
The flags portion of the XFER_ACK header SHALL be set to match the
corresponding DATA_SEGMENT message being acknowledged.
The acknowledged length of each XFER_ACK contains the sum of the
data length fields of all XFER_SEGMENT messages received so far in the
course of the indicated transfer.
The sending node SHOULD transmit multiple XFER_SEGMENT
messages without waiting for the corresponding
XFER_ACK responses.
This enables pipelining of messages on a transfer stream.
For example, suppose the sending node transmits four segments of
bundle data with lengths 100, 200, 500, and 1000, respectively.
After receiving the first segment, the node sends an acknowledgment
of length 100. After the second segment is received, the node sends
an acknowledgment of length 300. The third and fourth
acknowledgments are of length 800 and 1800, respectively.
The TCPCL supports a mechanism by which a receiving node can indicate to
the sender that it does not want to receive the corresponding bundle.
To do so, upon receiving an XFER_SEGMENT message, the node MAY
transmit a XFER_REFUSE message. As data segments and
acknowledgments MAY cross on the wire, the bundle that is being
refused SHALL be identified by the Transfer ID of the refusal.
There is no required relation between the Transfer MRU of a TCPCL
node (which is supposed to represent a firm limitation of what the
node will accept) and sending of a XFER_REFUSE message.
A XFER_REFUSE can be used in cases where the agent's bundle storage is
temporarily depleted or somehow constrained.
A XFER_REFUSE can also be used after the bundle header or any bundle data
is inspected by an agent and determined to be unacceptable.
A receiver MAY send an XFER_REFUSE message as soon as it receives
any XFER_SEGMENT message.
The sender MUST be prepared for this and MUST associate the refusal
with the correct bundle via the Transfer ID fields.
The format of the XFER_REFUSE message is as follows in
.
+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
The fields of the XFER_REFUSE message are:
A one-octet refusal reason code interpreted according to the
descriptions in
.
A 64-bit unsigned integer identifying the transfer being refused.
NameSemanticsUnknownReason for refusal is unknown or not specified.Extension FailureA failure processing the Transfer Extension Items ha occurred.CompletedThe receiver already has the complete bundle. The sender MAY consider the bundle as completely received.No ResourcesThe receiver's resources are exhausted. The sender SHOULD apply reactive bundle fragmentation before retrying.RetransmitThe receiver has encountered a problem that requires the bundle to be retransmitted in its entirety.
The receiver MUST, for each transfer preceding the one to be refused,
have either acknowledged all XFER_SEGMENTs or refused the bundle transfer.
The bundle transfer refusal MAY be sent before an entire data segment is
received. If a sender receives a XFER_REFUSE message, the sender
MUST complete the transmission of any partially sent XFER_SEGMENT
message. There is no way to interrupt an individual TCPCL message partway
through sending it.
The sender MUST NOT commence transmission of any further segments of the
refused bundle subsequently.
Note, however, that this requirement does not ensure
that an entity will not receive another XFER_SEGMENT for the same bundle
after transmitting a XFER_REFUSE message since messages MAY cross
on the wire; if this happens, subsequent segments of the bundle
SHALL also be refused with a XFER_REFUSE message.
Note: If a bundle transmission is aborted in this way, the receiver
MAY not receive a segment with the 'END' flag set to '1' for the
aborted bundle. The beginning of the next bundle is identified by
the 'START' bit set to '1', indicating the start of a new transfer, and with
a distinct Transfer ID value.
Each of the Transfer Extension Items SHALL be encoded in an identical
Type-Length-Value (TLV) container form as indicated in
.
The fields of the Transfer Extension Item are:
A one-octet field containing generic bit flags about the Item,
which are listed in
.
If a TCPCL node receives a Transfer Extension Item with an unknown Item Type
and the CRITICAL flag set, the node SHALL refuse the transfer
with an XFER_REFUSE reason code of "Extension Failure".
If the CRITICAL flag is not set, an entity SHALL skip over and ignore
any item with an unknown Item Type.
A 16-bit unsigned integer field containing the type of the extension item.
This specification allocates an IANA registry for such codes
(see
).
A 32-bit unsigned integer field containing the number of Item Value octets
to follow.
A variable-length data field which is interpreted according to the
associated Item Type.
This specification places no restrictions on an extension's use of
available Item Value data.
Extension specifications SHOULD avoid the use of large data lengths,
as the associated transfer cannot begin until
the full extension data is sent.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Item Flags | Item Type | Item Length...|
+---------------+---------------+---------------+---------------+
| length contd. | Item Value... |
+---------------+---------------+---------------+---------------+
| value contd. |
+---------------+---------------+---------------+---------------+
NameCodeDescriptionCRITICAL0x01If bit is set, indicates that the receiving peer must handle the extension item.Reservedothers
The purpose of the Transfer Length extension is to allow entities to preemptively refuse
bundles that would exceed their resources or to prepare storage on the
receiving node for the upcoming bundle data.
Multiple Transfer Length extension items SHALL NOT
occurr within the same transfer.
The lack of a Transfer Length extension item in any transfer
SHALL NOT imply anything about the potential length of the transfer.
The Transfer Length extension SHALL be assigned transfer extension type ID 0x0001.
The format of the Transfer Length data is as follows in
.
+----------------------+
| Total Length (U64) |
+----------------------+
The fields of the Transfer Length extension are:
A 64-bit unsigned integer indicating the size of the data-to-be-transferred.
The Total Length field SHALL be treated as authoritative by the receiver.
If, for whatever reason, the actual total length of bundle data
received differs from the value indicated by the
Total Length value, the receiver SHALL treat the transmitted data as invalid.
This section describes the procedures for ending a TCPCL session.
To cleanly shut down a session, a SESS_TERM message SHALL be
transmitted by either node at any point following complete
transmission of any other message.
When sent to initiate a termination, the REPLY bit of a SESS_TERM message
SHALL NOT be set.
Upon receiving a SESS_TERM message after not sending a SESS_TERM message in
the same session, an entity SHALL send an acknowledging SESS_TERM message.
When sent to acknowledge a termination, a SESS_TERM message SHALL have
identical data content from the message being acknowledged except for
the REPLY bit, which is set to indicate acknowledgement.
After sending a SESS_TERM message, an entity MAY continue a possible in-progress
transfer in either direction.
After sending a SESS_TERM message, an entity SHALL NOT begin any new outgoing
transfer (i.e. send an XFER_SEGMENT message) for the remainder of the session.
After receving a SESS_TERM message, an entity SHALL NOT accept any new incoming
transfer for the remainder of the session.
Instead of following a clean shutdown sequence, after transmitting a
SESS_TERM message an entity MAY immediately close
the associated TCP connection.
When performing an unclean shutdown, a receiving node SHOULD
acknowledge all received data segments before closing the TCP connection.
Not acknowledging received segments can result in unnecessary retransmission.
When performing an unclean shutodwn, a transmitting node SHALL treat
either sending or receiving a SESS_TERM message
(i.e. before the final acknowledgment) as a failure of the transfer.
Any delay between request to terminate the TCP connection and actual
closing of the connection (a "half-closed" state) MAY be ignored
by the TCPCL node.
The format of the SESS_TERM message is as follows in
.
+-----------------------------+
| Message Header |
+-----------------------------+
| Message Flags (U8) |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
The fields of the SESS_TERM message are:
A one-octet field of single-bit flags, interpreted according to the
descriptions in
.
A one-octet refusal reason code interpreted according to the
descriptions in
.
NameCodeDescriptionREPLY0x01If bit is set, indicates that this message is an acknowledgement of an earlier SESS_TERM message.ReservedothersNameDescriptionUnknownA termination reason is not available.Idle timeoutThe session is being closed due to idleness.Version mismatchThe node cannot conform to the specified TCPCL protocol version.BusyThe node is too busy to handle the current session.Contact FailureThe node cannot interpret or negotiate contact header option.Resource ExhaustionThe node has run into some resource limit and cannot continue the session.
A session shutdown MAY occur immediately after transmission of a
contact header (and prior to any further message transmit).
This MAY, for example, be used to notify
that the node is currently not able or willing to communicate.
However, an entity MUST always send the contact header to its peer
before sending a SESS_TERM message.
If reception of the contact header itself somehow fails (e.g. an invalid
"magic string" is recevied), an entity SHALL close the TCP connection
without sending a SESS_TERM message.
If the content of the Session Extension Items data disagrees with the
Session Extension Length (i.e. the last Item claims to use more octets
than are present in the Session Extension Length), the reception of the
contact header is considered to have failed.
If a session is to be terminated before a protocol message
has completed being sent, then the node MUST NOT transmit the SESS_TERM
message but still SHALL close the TCP connection.
Each TCPCL message is contiguous in the octet stream and has no ability
to be cut short and/or preempted by an other message.
This is particularly important when large segment sizes
are being transmitted; either entire XFER_SEGMENT is sent before a SESS_TERM
message or the connection is simply terminated mid-XFER_SEGMENT.
The protocol includes a provision for clean shutdown of idle
sessions. Determining the length of time to wait before closing
idle sessions, if they are to be closed at all, is an
implementation and configuration matter.
If there is a configured time to close idle links and if no TCPCL messages
(other than KEEPALIVE messages) has been received for at least
that amount of time, then either node MAY terminate the session by
transmitting a SESS_TERM message indicating the reason code of "Idle
timeout" (as described in
).
[NOTE to the RFC Editor: please remove this section before
publication, as well as the reference to
and
.]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of
this Internet-Draft, and is based on a proposal described in
.
The description of implementations in this section is
intended to assist the IETF in its decision processes in progressing
drafts to RFCs. Please note that the listing of any individual
implementation here does not imply endorsement by the IETF.
Furthermore, no effort has been spent to verify the information
presented here that was supplied by IETF contributors. This is not
intended as, and must not be construed to be, a catalog of available
implementations or their features. Readers are advised to note that
other implementations may exist.
An example implementation of the this draft of TCPCLv4 has been
created as a GitHub project
and is intented to use as a proof-of-concept and as a possible source
of interoperability testing.
This example implementation uses D-Bus as the CL-BP Agent interface,
so it only runs on hosts which provide the Python "dbus" library.
One security consideration for this protocol relates to the fact that
entities present their endpoint identifier as part of the contact
header exchange. It would be possible for an entity to fake this value
and present the identity of a singleton endpoint in which the node is
not a member, essentially masquerading as another DTN node. If this
identifier is used outside of a TLS-secured session or
without further verification as a means to
determine which bundles are transmitted over the session, then the
node that has falsified its identity would be able to obtain bundles
that it otherwise would not have.
Therefore, an entity SHALL NOT use the EID value of an unsecured contact header
to derive a peer node's identity unless it can corroborate it via other
means.
When TCPCL session security is mandated by a TCPCL peer, that peer SHALL
transmit initial unsecured contact header values indicated in
in order.
These values avoid unnecessarily
leaking session parameters and will be ignored when secure contact header
re-exchange occurs.
ParameterValueFlagsThe USE_TLS flag is set.Keepalive IntervalZero, indicating no keepalive.Segment MRUZero, indicating all segments are refused.Transfer MRUZero, indicating all transfers are refused.EIDEmpty, indicating lack of EID.
TCPCL can be used to provide point-to-point transport security, but does
not provide security of data-at-rest and does not guarantee
end-to-end bundle security.
The mechanisms defined in
and
are to be used instead.
Even when using TLS to secure the TCPCL session, the actual ciphersuite
negotiated between the TLS peers MAY be insecure.
TLS can be used to perform authentication without data confidentiality,
for example.
It is up to security policies within each TCPCL node to ensure that the
negotiated TLS ciphersuite meets transport security requirements.
This is identical behavior to STARTTLS use in
.
Another consideration for this protocol relates to denial-of-service
attacks. An entity MAY send a large amount of data over a TCPCL
session, requiring the receiving entity to handle the data, attempt
to stop the flood of data by sending a XFER_REFUSE message, or
forcibly terminate the session. This burden could cause denial of
service on other, well-behaving sessions. There is also nothing
to prevent a malicious entity from continually establishing sessions
and repeatedly trying to send copious amounts of bundle data. A
listening entity MAY take countermeasures such as ignoring TCP SYN
messages, closing TCP connections as soon as they are established,
waiting before sending the contact header, sending a SESS_TERM message
quickly or with a delay, etc.
In this section, registration procedures are as defined in
.
Some of the registries below are created new for TCPCLv4 but share code
values with TCPCLv3. This was done to disambiguate the use of these values
between TCPCLv3 and TCPCLv4 while preserving the semantics of some values.
Port number 4556 has been previously assigned as the default port for the
TCP convergence layer in
.
This assignment is unchanged by protocol version 4.
Each TCPCL entity identifies its TCPCL protocol version in its initial
contact (see
), so there is no ambiguity about what protocol is being used.
ParameterValueService Name:dtn-bundleTransport Protocol(s):TCPAssignee:Simon Perreault <simon@per.reau.lt>Contact:Simon Perreault <simon@per.reau.lt>Description:DTN Bundle TCP CL ProtocolReference:Port Number:4556
IANA has created, under the "Bundle Protocol" registry, a sub-
registry titled "Bundle Protocol TCP Convergence-Layer Version
Numbers" and initialize it with the following table.
The registration procedure is RFC Required.
ValueDescriptionReference0Reserved1Reserved2Reserved3TCPCL4TCPCLbisThis specification.5-255UnassignedEDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry,
a sub-registry titled
"Bundle Protocol TCP Convergence-Layer Version 4 Session Extension Types"
and initialize it with the contents of
.
The registration procedure is RFC Required within the lower range 0x0001--0x7fff.
Values in the range 0x8000--0xffff are reserved for use on private networks
for functions not published to the IANA.
CodeMessage Type0x0000Reserved0x0001--0x7fffUnassigned0x8000--0xffffPrivate/Experimental UseEDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry,
a sub-registry titled
"Bundle Protocol TCP Convergence-Layer Version 4 Transfer Extension Types"
and initialize it with the contents of
.
The registration procedure is RFC Required within the lower range 0x0001--0x7fff.
Values in the range 0x8000--0xffff are reserved for use on private networks
for functions not published to the IANA.
CodeMessage Type0x0000Reserved0x0001Transfer Length Extension0x0002--0x7fffUnassigned0x8000--0xffffPrivate/Experimental UseEDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry,
a sub-registry titled
"Bundle Protocol TCP Convergence-Layer Version 4 Message Types"
and initialize it with the contents of
.
The registration procedure is RFC Required.
CodeMessage Type0x00Reserved0x01XFER_SEGMENT0x02XFER_ACK0x03XFER_REFUSE0x04KEEPALIVE0x05SESS_TERM0x06MSG_REJECT0x07--0xfUnassignedEDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry,
a sub-registry titled
"Bundle Protocol TCP Convergence-Layer Version 4 XFER_REFUSE Reason Codes"
and initialize it with the contents of
.
The registration procedure is RFC Required.
CodeRefusal Reason0x0Unknown0x1Extension Failure0x2Completed0x3No Resources0x4Retransmit0x5--0x7Unassigned0x8--0xfReserved for future usageEDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry,
a sub-registry titled
"Bundle Protocol TCP Convergence-Layer Version 4 SESS_TERM Reason Codes"
and initialize it with the contents of
.
The registration procedure is RFC Required.
CodeShutdown Reason0x00Unknown0x01Idle timeout0x02Version mismatch0x03Busy0x04Contact Failure0x05Resource Exhaustion0x06--0xFFUnassignedEDITOR NOTE: sub-registry to-be-created upon publication of this specification.
IANA will create, under the "Bundle Protocol" registry,
a sub-registry titled
"Bundle Protocol TCP Convergence-Layer Version 4 MSG_REJECT Reason Codes"
and initialize it with the contents of
.
The registration procedure is RFC Required.
CodeRejection Reason0x00reserved0x01Message Type Unknown0x02Message Unsupported0x03Message Unexpected0x04-0xFFUnassigned
This specification is based on comments on implementation of
provided from Scott Burleigh.
Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application
protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the last few years, several serious attacks on TLS have emerged, including
attacks
on its most commonly used cipher suites and their modes of operation. This document provides recommendations for improving the security of
deployed services that use TLS and DTLS. The recommendations are applicable to the majority of use cases.TCPCL Example Implementation
RKF Engineering Solutions, LLC
The areas in which changes from
have been made to existing headers and messages are:
Split contact header into pre-TLS protocol negotiation and SESS_INIT parameter negotiation. The contact header is now fixed-length.Changed contact header content to limit number of negotiated options.Added contact option to negotiate maximum segment size (per each direction).Added session extension capability.Added transfer extension capability. Moved transfer total length into an extension item.Defined new IANA registries for message / type / reason codes to allow renaming some codes for clarity.Expanded Message Header to octet-aligned fields instead of bit-packing.Added a bundle transfer identification number to all bundle-related messages (XFER_SEGMENT, XFER_ACK, XFER_REFUSE).Use flags in XFER_ACK to mirror flags from XFER_SEGMENT.Removed all uses of SDNV fields and replaced with fixed-bit-length fields.Renamed SHUTDOWN to SESS_TERM to deconflict term "shutdown".Removed the notion of a re-connection delay parameter.
The areas in which extensions from
have been made as new messages and codes are:
Added contact negotiation failure SESS_TERM reason code.Added MSG_REJECT message to indicate an unknown or unhandled message was received.Added TLS session security mechanism.Added Resource Exhaustion SESS_TERM reason code.